John Heil, Professor of Philosophy, Washington University in St Louis, gives a talk on Cartesian Transubstantiation. According to the received view of the metaphysics of the Eucharist endorsed by the Catholic Church after the thirteenth century, sacramental bread and wine are 'converted' into Christ's body and blood (this is transubstantiation), but the accidents of the bread and wine remain on the altar inhering in no substance. Such a view is difficult to square with Aristotelian physics, but much more difficult to reconcile with the physics of Descartes. Two ill-fated attempts by Descartes to provide an account of transubstantiation consistent with his conception of the material universe are discussed in the context of a broader discussion of related metaphysical issues.